1. Field of the Invention
The invention concerns a method for repairing transmission masks for X-ray, electron and ion beam lithography, respectively.
2. Background of the Invention
The continuing trend of VLSI semiconductor technology towards reduced circuit dimensions calls for lithographic processes and exposure systems permitting the printing of submicrometer patterns. Due to the low focussing depth and the scattering of light by the mask edges, previously used photooptical exposure systems are no longer suitable for producing such submicrometer patterns. Therefore, methods have to be adopted using non-optical radiations with much shorter wavelengths than those employed in photooptical systems. Non-optical radiations currently used are X-rays, electron or ion beams.
X-ray, electron or ion beam exposure necessitates transmission masks with submicrometer structures and a high degree of geometrical precision which is essential for overlay printing. In contrast to masks for optical exposure comprising chromium structures on glass substrates, the masks for electron and ion beam lithography are made up of thin silicon or silicon nitride foils or membranes with physical holes in their transparent regions for passing the radiation. They are manufactured photolithographically followed by isotropic and strictly anisotropic etching. During manufacture, dust particles or, even more frequently, the formation of holes in the photoresist to be exposed may lead to additional undesired holes. In addition, lattice defects occurring during the boron doping of the wafers making up the masks may lead to further defect holes during selective etching. Such defect holes are undesirable because they are etched in the same way as the desired structures during the manufacture of the transmission mask, thus falsifying the mask structures. Even optimum clean room conditions fail to completely avoid such additional holes. To remedy this, methods have been developed for repairing transmission masks.
The mask repair process used in photooptical lithography, wherein excessive chromium is removed from the mask structures by a laser beam or defect holes are closed by manually applying photoresist with a small brush, is unsuitable for repairing the above-described transmission masks, since, unlike photooptical masks, the repair process is not performed on samples that have been magnified up to 10 times their natural size but on transmission masks with structures at a 1:1 scale, with the resultant structures being frequently smaller than 0.5 .mu.m.
IBM Technical Disclosure Bulletin, Vol. 27, No. 8, January 1985, pp. 4815, 4816, describes a method for repairing defective transmission masks, wherein a photoresist layer is applied to the top side of the defective mask and all mask holes, including the defective ones, are closed. The photoresist layer is subsequently exposed through a transmission mask, whose structure is identical to that of the mask to be repaired, taking care that the positions of defect holes, if any, in the exposure mask do not coincide with those in the mask to be repaired. Further defects, resulting from defective regions of the mask used for repair are eliminated by reexposing the photoresist layer on the mask to be repaired through a mask of the same structure as that used in the first exposure step, once again taking care that the positions of the defect hole of the mask used for reexposure do not coincide with those of the previously used mask and the mask to be repaired. This method has the disadvantage that the photoresist remaining on the mask may lead to undesirable charges when the mask is subsequently exposed. In addition, this method provides for repatterning of the entire mask. This described repair process is recommendable when there are many defect holes to be closed. If as a consequence of optimum clean room conditions visual mask inspection reveals only very few defect holes to be closed, it may be more advantageous to use a method permitting the repair of single defect holes. However, known methods for repair of single defect holes take several minutes to repair a single hole so that mask repair time can become intolerable.
It is the object of the present invention to provide a method for repairing transmission masks, which permits all mask patterns to be repaired, regardless of the number of defects, to their desired patterns within an economically acceptable time.
It is another object of the present invention to provide a repaired mask which does not lead to undesirable charge build-up when the mask is subsequently used in lithographic processing.